I think what you are asking for is more of a Front End software issue then a HD Radio one. I know that almost all can do multizone but all handle it differently. I am no expert but I realy dont think your asking in the correct place. I think you need to decide on a front end CF3, SD2, RideRunner or OpenMobile (for windows anyway). Then ask the same question in that forum. Some one correct me if I am wrong.
customrv, what dave says it a bit right, i do know audio is a complication issue with CARPCs... im not 100% sure how to help you here
its possible that no Front End alone will solve your issues...
and maybe something like AudioMulch will need to come into play
there is a bunch of talk in the forums(somewhere) about advanced audio
me, i might want it all externally processed... im not sure exactly how i would want it
(i say would, because i dont have a carpc)
this is pretty easy to do.... grab of copy of virtual audio cable (google it) and create two virtual sound cards...one mapped to the front speakers, one mapped to the rear. Then openMobile or CF should be able to pull it off.